Now that we have a better understanding of what goes on in a basic 12-bar blues and two ways to color this tried-and-true progression, let's start to play around with its content. First up is adding a single chord to the mix in the form of a dim7 chord. Let's drop the same one--F#dim7--in bars 2, 6 and 10 following the IV chord (F7).

The logic behind this chord is, on the surface, pretty simple. You're just raising the root of the IV chord up a half step. Besides sounding totally cool, this makes for good, tight voice leading and more interesting bass movement. The piquant color of the dim7 is a welcome tension that is resolved every time by the tonic I7 chord.

The #ivdim7 chord is a sound that's easily recognizable. Just take a listen to your favorite blues CD's and I guarantee you'll discover countless examples of this chordal idea. Keep an eye, err, ear open for this cool tool as you progress through the course, as it will be one that pops up again and again.